In recent years, a large-scale integrated circuit (hereinafter, referred to as LSI) implemented with a semiconductor has been increasingly reduced in size. As a result, a feature error or dimensional error between a mask pattern and a produced pattern (e.g., a resist pattern formed by pattern transfer for a resist film) have been increasingly regarded as important in a lithography process, one of the LSI manufacturing processes.
Moreover, reduction in pattern dimension in the LSI has reached about the resolution limit defined by a wavelength of a light source (hereinafter, referred to as wavelength λ), a numerical aperture of a projection optical system of an aligner (hereinafter, referred to as numerical aperture NA), and the like. As a result, a manufacturing margin associated with the yield in LSI manufacturing, e.g., a depth of focus, has also been significantly reduced.
In a conventional patterning method, a resist pattern having a prescribed feature is formed as follows: a light-shielding pattern of a prescribed feature, i.e., a mask pattern, is formed on a transparent substrate using a light-shielding film of a metal such as chromium. Then, a wafer having a resist film applied thereto is exposed to light using the transparent substrate having the mask pattern thereon as a mask, so that light intensity distribution having a profile similar to the mask pattern feature is projected to the resist film. Thereafter, the resist film is developed, whereby the resist pattern having the prescribed feature is produced.
A reduction projection aligner is generally used in such a patterning method as described above. For patterning, the reduction projection aligner conducts reduction projection exposure for a resist film of a photosensitive resin formed on a wafer, i.e., a substrate, by using a transparent substrate including a mask pattern with the dimension of a desired pattern magnified several times, i.e., by using a photomask.
FIG. 32(a) shows an example of a pattern whose minimum dimension is sufficiently larger than the resolution. FIG. 32(b) shows the simulation result of light intensity distribution projected to, e.g., a resist film upon forming the pattern of FIG. 32(a) using a conventional photomask.
More specifically, when the numerical aperture NA is 0.6 and the wavelength λ is 0.193 μm, the resolution is about 0.13 μm. However, the minimum dimension of the pattern of FIG. 32(a) is about 0.39 μm (about three times the resolution). The conventional photomask has a mask pattern having the dimension of the pattern of FIG. 32(a) magnified by the magnification M of the aligner (an inverse number of a reduction ratio). In this case, as shown in FIG. 32(b), the implemented light intensity distribution has a profile similar to the feature of the pattern of FIG. 32(a), i.e., the mask pattern. Note that FIG. 32(b) shows the light intensity distribution using contour lines of the relative light intensity in a two-dimensional relative coordinate system (i.e., the light intensity calculated with the exposure light intensity being regarded as 1).
FIG. 33(a) shows an example of a pattern whose minimum dimension corresponds to about the resolution. FIG. 33(b) shows the simulation result of light intensity distribution projected to, e.g., a resist film upon forming the pattern of FIG. 33(a) using a conventional photomask.
More specifically, when the numerical aperture NA is 0.6 and the wavelength λ is 0.193 μm, the resolution is about 0.13 μm. The minimum dimension of the pattern of FIG. 33(a) is also about 0.13 μm. The conventional photomask has a mask pattern having the dimension of the pattern of FIG. 33(a) magnified by the magnification M. In this case, as shown in FIG. 33(b), the implemented light intensity distribution is significantly distorted from the profile similar to the feature of the pattern of FIG. 32(a), i.e., the mask pattern. Note that FIG. 33(b) also shows the light intensity distribution using contour lines of the relative light intensity in a two-dimensional relative coordinate system.
More specifically, as the minimum dimension of the pattern is reduced to about the resolution, the line width of the mask pattern on the photomask is also reduced. Therefore, the exposure light is likely to be diffracted when passing through the photomask. More specifically, as the line width of the mask pattern is reduced, the exposure light is likely to reach the backside of the mask pattern. As a result, the mask pattern cannot sufficiently shield the exposure light, making it extremely difficult to form a fine pattern.
In order to form a pattern having a dimension equal to or smaller than about the resolution, H. Y. Liu et al. proposes a patterning method (first conventional example) (Proc. SPIE, Vol. 3334, P.2 (1998)). In this method, a light-shielding pattern of a light-shielding film is formed on a transparent substrate as a mask pattern, as well as a phase shifter for inverting the light transmitted therethrough by 180 degrees in phase is provided in a light-transmitting region (a portion having no light-shielding pattern) of the transparent substrate. This method utilizes the fact that a pattern having a dimension equal to or smaller than about the resolution can be formed by the light-shielding film located between the light-transmitting region and the phase shifter.
Hereinafter, the patterning method according to the first conventional example will be described with reference to FIGS. 34(a) to (d).
FIG. 34(a) is a plan view of a first photomask used in the first conventional example, and FIG. 34(b) is a cross-sectional view taken along line I—I of FIG. 34(a). As shown in FIGS. 34(a) and (b), a light-shielding film 11 is formed on a first transparent substrate 10 of the first photomask, and first and second openings 12 and 13 are formed in the light-shielding film 11 such that a light-shielding film region 11a having a width smaller than (resolution×magnification M) is interposed therebetween. The first transparent substrate 10 is recessed under the second opening 13 so as to provide a phase difference of 180 degrees between the light transmitted through the first transparent substrate 10 through the first opening 12 and the light transmitted through the first transparent substrate 10 through the second opening 13. Thus, the portion of the first transparent substrate 10 corresponding to the first opening 12 serves as a normal light-transmitting region, whereas the portion of the first transparent substrate 10 corresponding to the second opening 13 serves as a phase shifter. Therefore, a pattern having a desired line width equal to or smaller than about the resolution can be formed by the light-shielding film region ha located between the first and second openings 12 and 13.
FIG. 34(c) is a plan view of a second photomask used in the first conventional example. As shown in FIG. 34(c), a light-shielding pattern 21 of a light-shielding film is formed on a second transparent substrate 20 of the second photomask.
In the first conventional example, a desired pattern is formed by combination of a line pattern formed by the light-shielding film region 11a of the first photomask of FIG. 34(a) and a pattern formed by the light-shielding pattern 21 of the second photomask of FIG. 34(c).
More specifically, in the first conventional example, a substrate having a positive resist film applied thereto is exposed to light using the first photomask of FIG. 34(a). Then, the substrate is adjusted in position so that a desired pattern is formed by a latent image resulting from exposure using the first photomask and a latent image resulting from exposure using the second photomask of FIG. 34(c). After exposure is subsequently conducted using the second photomask, the resist film is developed, whereby a resist pattern is formed. Thus, excessive patterns (patterns other than the desired pattern) resulting from development after exposure with the first photomask only can be removed by exposure with the second photomask. This enables formation of a pattern having a line width equal to or smaller than about the resolution, i.e., a pattern that cannot be formed by exposure with the second photomask only.
FIG. 34(d) shows a resist pattern formed by the patterning method of the first conventional example, i.e., the patterning method using the first and second photomasks of FIGS. 34(a) and 34(c).
As shown in FIG. 34(d), the exposed substrate 30 has a resist pattern 31 formed thereon, and the resist pattern 31 has a line pattern 31a having a line width equal to or smaller than about the resolution.
In addition to the method of H. Y. Liu et al., Watanabe et al. proposes another patterning method (second conventional example) (Proc. of the 51st Annual Meeting of JSAP, P 490). In this method, a pattern having a line width smaller than the wavelength λ is formed without providing a light-shielding film between a light-transmitting region and a phase shifter. This method utilizes the effect that a pattern is formed by the boundary between a normal transparent substrate portion, i.e., a light-transmitting region, and a phase shifter.
Hereinafter, the patterning method according to the second conventional example will be described with reference to FIG. 35.
FIG. 35 is a plan view of a photomask used in the second conventional example. As shown in FIG. 35, a plurality of phase shifters 41 are periodically arranged on a transparent substrate 40 of the photomask.
In the second conventional example, the use of the phase shifters 41 enables formation of a pattern in which a plurality of line patterns each having a line width smaller than the wavelength λ are arranged periodically.
However, in order to form a pattern having a line width equal to or smaller than about the resolution, the first conventional example must use a phase shift mask (first photomask) in which a light-shielding film region having a width of (resolution×magnification M) or less is located between a phase shifter and a light-transmitting region both having a width of (resolution×magnification M) or more. In other words, the pattern formed with the first photomask has a line width equal to or smaller than about the resolution only when specific conditions are satisfied. Therefore, an arbitrary pattern feature cannot be implemented with the first photomask only.
Accordingly, in order to form a pattern having a complicated feature like in the pattern layout of a normal LSI, exposure with a mask (second photomask) different from the phase shift mask is essential in the first conventional example. This results in increase in mask costs, or reduction in throughput as well as increase in manufacturing costs due to an increased number of lithography steps.
Moreover, a normal mask, i.e., a non-phase-shift mask, is used as the second photomask. Therefore, even if the exposures using the first and second photomasks are combined, the pattern formed by the second photomask has a dimension equal to or larger than about the resolution, whereby the patterns capable of being formed with a dimension equal to or smaller than about the resolution are limited. In other words, the first conventional example is used only when the phase shifter and the light-transmitting region can be located adjacent to each other under the aforementioned conditions, e.g., when only a gate pattern on an active region is formed.
In contrast, the second conventional example, i.e., the method in which a pattern is formed without providing a light-shielding film between a light-transmitting region and a phase shifter, can be used only when the patterns each having a line width smaller than the wavelength λ are repeated. Therefore, a pattern having an arbitrary feature or an arbitrary dimension cannot be formed by this method alone.
Moreover, in the second conventional example, a portion where the phase changes abruptly must be provided at the boundary between the light-transmitting region of the transparent substrate and the phase shifter. However, by the conventional mask formation method in which a phase shifter is formed by wet etching the transparent substrate, the transparent substrate cannot be etched vertically at the boundary of the phase shifter. Moreover, when the transparent substrate is etched, a lateral region of the phase shifter in the transparent substrate is also subjected to etching, making it difficult to control the dimension of the phase shifter. As a result, it is extremely difficult to produce a mask capable of forming a fine pattern with high precision.
In the second conventional example, the dimension of the pattern formed by utilizing the phase shift effect is limited to about half the wavelength λ. However, when a pattern having a larger dimension is formed with a mask pattern of a light-shielding film, the minimum possible dimension of the pattern corresponds to about the resolution. Accordingly, in the case where patterning is conducted using a single mask that simultaneously implements the phase shift effect and the light-shielding effect of the light-shielding film, a possible dimensional range of the pattern is discontinuous. This significantly reduces a process margin for forming a pattern of an arbitrary dimension with a single mask, and in some cases, makes it impossible to form a pattern with a single mask.